genfs_contexts

Rename sdcard_internal/external types to fuse and vfat
respectively to make it clear that they are assigned to any
fuse or vfat filesystem by default (absent a context= mount option)
and do not necessarily represent the SDcard.

The sdcard_type attribute is still assigned to both types and
can still be used in allow rules to permit access to either the
internal or external SDcard.

Define type aliases for the old names to preserve compatibility
on policy reload and for device-specific policies that may not yet
be updated.

Change-Id: I8d91a8c4c1342b94e4f1bb62ca7ffd2ca4b06ba1
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>